North Hawaii News Articles from CFHT

Astronomical Distances - Tentative First Steps

It is a funny fact in astronomy that one of the most difficult measurements to make is something that seems trivial: the distance to a star. Astronomers can easily measure the temperature of a star, the chemical composition, the velocity of a star (at least in one direction), even the amount of methane between us and the star -- these are all pretty easy and accurate. But the distance is difficult. We all know how to measure a distance: just place a ruler down and look at the tick marks. When you can't reach a star, it is not so easy.

After many years of work, we are now able to measure distances to objects billions of times further away than the nearest star, which is in turn almost a million times further away than the Sun. Getting to this point has been a long (very long!) effort, with many important intermediate steps along the way. The history of these discoveries is largely the history of Astronomy itself.

Astronomers use many different types of measurements to determine distances to celestial objects, depending on the how far the object is. We talk about the 'distance ladder' because we use measurements to nearby objects to get the distance to further objects. Then we use the distance to the distant objects to get even further ones. The entire process is just a bit too complex for a single article, so I'll split the ladder in pieces. In this week's article, I'll talk about the first steps of measuring astronomical distances. In the next article, I'll talk about the race to find the distance to the planets and the closest stars. Finally, I'll talk about bridging the gap from the nearest stars to the most distant galaxies.

Let's start with the easiest measurement: the size of the Earth. High school science text books sometimes mention Eratostheses as the first person to make a good measurement of the Earth's diameter. Eratosthenes was the librarian of the great Library of Alexandria around 250 BC. He had access to one of the largest collections of knowledge ever available up to that time (sort of like an early World Wide Web...).

While reading in the library, Eratosthenes discovered that, on a certain day in the year, the sun was directly overhead Syene, a city in the far south of Egypt. He knew the distance to Syene from other books in the library. By measuring the angle of the sun in Alexandria on the right day, he found that the Earth is about 7850 miles in diameter, very close to our modern number of 7900 miles. To be fair, Eratosthenes was not the first to try this measurement. The concept of a round Earth had been around for 200 years; Aristotle and other Greeks tried the same trick using stars, but they never got a very accurate number.

Not long after Eratosthenes, another Greek astronomer measured the distance to the Moon and made a good attempt to measure the distance to the Sun. Aristarchus watched the shadow of the Earth on the Moon during a lunar eclipse. He realized that the angle of the shadow on the sky, coupled with the Earth's diameter told him the distance to the Moon. He came up with a number of 180,000 miles, only somewhat smaller than the actual 250,000 miles.

Knowing the distance to the Moon, Aristarchus tried to measure the distance to the Sun. He measured the angle between the Sun and Moon when the Moon was exactly half lit by the Sun. The idea is that the Moon and Earth see the Sun at slightly different angles, and this angle combined with the distance to the Moon gives the distance to the Earth. His distance to the Sun was 4 million miles, almost 25 times smaller than the real distance. We have to give him some slack because it is a very hard measurement to make with primitive equipment. Even so, this large distance told him that the Sun was much larger than the Earth. Based on this, he proposed that the Earth orbited the Sun, instead of the then popular theory that the Sun orbited the Earth. His theory never caught on - Archimedes mentioned it, but only as a curious suggestion. Advances in understanding the layout of the Solar System and the Universe had to wait another 1750 years for Nicolaus Copernicus.

If you are impatient to read the rest of this series, or if you want to see past astronomy articles from North Hawaii News, we have compiled them on our web page:

Eugene Magnier